1. Field of the Invention
This invention relates to an ignition distributor for internal combustion engines, comprising a housing, in which a rotor of the distributor is rotatably mounted, which is provided near its axis of rotation with a first electrode and at an end that is remote from its axis of rotation with a second electrode and comprises an electric line connecting the two electrodes, and a distributor cap, which closes the housing and is provided on its inside surface with a plurality of peripherally spaced apart fixed electrodes for cooperating with the second electrode of the rotor, and with a center electrode, wherein said fixed electrodes and center electrode are embedded in said cap and said center electrode terminates at a distance from the first electrode of the rotor to define a spark gap with said first electrode. The center electrode has a portion which is embedded in said cap, and said embedded portion has a non-cylindrical peripheral surface for establishing a positive joint with said cap.
2. Description of the Prior Art
In modern internal combustion engines the ignition distributor is directly mounted on the engine block, which thus constitutes the base part of the distributor housing, in which the rotor of the distributor is rotatably mounted. The top part of the distributor housing usually consists of a cap, which is made of a synthetic resin, usually a polyester or epoxide resin. That distributor cap carries on its inside surface a plurality of peripherally spaced apart, fixed electrodes and a center electrode, from which the igniting voltage is applied to the electrode of the rotor so that a flashover is effected from the rotor electrode through a spark gap to the fixed electrodes.
Because the distributor is mounted directly on the engine block the distributor is exposed in operation to a high temperature of about 130.degree. C. and all elements of the distributor must withstand that temperature in long-time operation. The fact that the high operating temperature has undesired consequences has become apparent when the center electrode of the distributor cap initially consisted of a sliding carbon electrode, which was resiliently urged against the confronting electrode of the rotor, in the arrangement which is usual in distributors that are spaced from the engine block. It has been found that the sliding carbon electrode is liable to break at the high operating temperatures. For this reason it has been proposed in DE-37 43 940 A1 to provide a distributor which is of the kind described first hereinbefore and in which the igniting voltage is applied to the rotor of the distributor via a spark gap that is provided between the rotor and the cap rather than via a sliding contact. In that case the center electrode may be fixedly embedded in the cap of the distributor and may be made of metal. Whereas the need for sliding carbon electrodes which are liable to break is thus eliminated, new problems will arise. If the voltage between the center electrode and the electrode of the rotor is about 35 kV, the sparks between said electrodes will result in a considerable stress on the electrodes and on the plastic material in which the electrodes are embedded. Each spark discharge will release energy, by which the electrodes and the surrounding plastic material are heated. At the same time chemically reactive compounds are formed in the ignition distributor atmosphere. Spark discharges will also occur between the rotor and the fixed electrodes because a spark gap for applying the igniting voltage has always been provided there. But that fact has not given rise to problems regarding the fixed electrodes in the past because a spark discharge occurs at said fixed electrodes only once in each revolution of the rotor. On the other hand, there are a plurality of spark discharges--equal in number to the cylinders of the engine--at the center electrode during each revolution of the rotor so that the wear of the center electrode of the distributor cap will be a multiple of the wear of the fixed electrodes. This is aggravated by the fact that a distributor which is directly mounted on the engine block will always be at a high temperature during operation and that the plastic material of which the cap is made is stressed at the same time by a high voltage up to 35 kilovolts. As a result, cracks will be formed in the plastic material which surrounds the center electrode and will become visible after a travel of about 10,000 kilometers. The cracks will become wider and deeper thereafter and portions of the plastic material may crumble and break out of the cap. In order to avoid such results it has been proposed by the makers of motor vehicles that the cap and the rotor of the distributor should be replaced after a travel of 10,000 km although this involves higher servicing costs.